Network analyzers are an essential tool for characterizing radio frequency devices. Network analyzers are often embedded into systems for characterizing antennas, radar cross section, propagation paths, materials sensors etc. The common structure of a network analyzer is a frequency-stepped signal source, multiple receivers, where at least one of the multiple receivers measures a reference signal, and at least one of the receivers simultaneously measures a signal arriving from a device under test (DUT). The dwell time on each frequency depends on the amount of signal averaging desired, affecting the measurement accuracy and sensitivity, and it is reflected in a “IF bandwidth” or “resolution bandwidth” parameter. The number of frequencies over which the sweep is performed affects the overall sweep time.
The DUT parameters are usually assumed to remain constant throughout the sweep time. In most applications this does not pose a limitation, such as when characterizing passive networks, e.g. filters. However, in many cases, the network parameters change over time. For example, antenna may rotate in an antenna range. An amplifier may warm up and change its characteristics after turn-on. Material under test may move on a conveyor belt or in a pipe, each time bringing in a new sample. A patient may breathe or move during examination in a medical microwave imaging system. An indoor propagation path may vary due to plasma discharge buildup and decay in a fluorescent lamp. In such cases, long acquisition time poses a limitation. It is, therefore, desirable to have a network analyzer with a substantially shorter acquisition time, capable of characterizing networks and devices in real time.
In some applications network parameters vary over time in a manner that precludes the use of conventional swept frequency network analyzers. Swept measurements incur penalty both in terms of acquisition time, and in terms of registration between measurements taken at the beginning and at the end of a sweep.